1. Field of the Invention
The present invention relates to a device for controlling a thermal printing head to produce multiple levels of gradation in each dot, and relates also to a printing apparatus using the thermal printing head.
This application is based on a patent application No. Hei 11-232025 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
In printers using a thermal printing head, because each dot is responsible for creating its own shading tone, each heating element is controlled so as to vary the duration of application of heat to each heating element so that each dot can produce a required degree of tone in a printed image (or printed letter). Methods for controlling conventional thermal printing heads to produce multi-level gradation are based on one of the two control methods outlined below.
(1) Method Based on Switching Head Data as Many Times as the Gradation Levels
This method is based on the use of a thermal printing head (referred to as thermal head hereinbelow) in which each dot is given its own register containing one bit. Heat-on or heat-off instruction (xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d) is sent to the thermal head from an external device to control the heating duration for each dot. In this method, if there are 256 levels of gradation, for example, heat on/off instructions must be given at least 256 times or more to each heating element to produce appropriate levels of printed tone.
FIG. 5 shows a block diagram of an example of the circuit to provide a 1-bit register for each dot in the thermal head. In FIG. 5, each heating element corresponds to a dot, so that a thermal head is comprised by heating elements R1xcx9cR1920 representing 1920 dots aligned to produce one line of printed dots; 15 integrated control circuits IC1xcx9cIC15, each of which controls on/off heating of 128 heating elements; and a thermister for determining the temperature of the thermal head.
As indicated in the block circuit diagram shown in FIG. 6, each of the integrated circuits IC1xcx9cIC15 is comprised by: a 128-bit (rows) shift register containing 128 steps of D flip-flop circuits SFF1xcx9cSFF128, connected in series, for receiving data from any one of the data signals DATA1xcx9cDATA 15; latching circuits comprised by D flip-flop circuits LFF1xcx9cLFF128 for holding the output of each row of shift registers SFF1xcx9cSFF128; and 128 pieces of NAND gating circuits G1xcx9cG128 controlled by strobe signal STROBE; so that each integrated circuit controls application of on/off voltage pulses from an electrical source VH to 128 dots according to the data (xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d) held in the register. The shift register is operated by shift clock signals CLOCK shown in FIG. 5, and an inverted signal produced by inverting the strobe signal STROBE in an inverter INV1 is input in the input terminal of each of the NAND-circuits G1xcx9cG128, and an inverted signal produced by inverting the latch signal LATCH in an inverter INV2 is input in the flip-flop circuits FF1xcx9cFF128 for timing.
In the method that uses the circuits shown in FIGS. 5, 6 to switch the head data as many times as the number of levels to express the required degree of shading, a data string containing a set of data of 128-dots each, consisting of 128 individual bits to control on/off values of each dot, is input simultaneously (in parallel) from each of the data lines DATA1xcx9cDATA15 to the respective integrated circuits IC1xcx9cIC15 successively in synchronization with the clock signals. In the integrated circuits IC1xcx9cIC15, the data are shifted in the shift registers SFF1xcx9cSFF128, and when all the 128-bits are transferred into the shift registers, the bit-data in the shift registers are latched by the latching circuits LFF1xcx9cLFF128 when the latch signal is xe2x80x9c0xe2x80x9d. When the strobe signal STROBE turns to xe2x80x9c0xe2x80x9d, latched data in the latching circuits are output from the integrated circuits IC1xcx9cIC15 so as to apply the latched data to those thermal heads showing xe2x80x9c1xe2x80x9d. When the strobe signal STROBE turns to xe2x80x9c1xe2x80x9d, all the heating elements are switched off by the Integrated circuits IC1xcx9cIC15.
Here, fifteen data signal lines DATA1xcx9cDATA15, and output lines for clock signals, latch signal LATCH, strobe signal STROBE, thermister TH1 are connected to the central processor device disposed within the printer (not shown).
A specific example of data transfer will be used to examined the process of data transfer in such a control device. Suppose that there are 1024-levels of gradations to be controlled at a data transfer rate of 14 MHz (frequency of clock signal CLOCK). To control 1024-levels of gradations, it is necessary for each data line to switch on/off data for each heating element in the thermal head 1024-times. The time required for printing one line is determined by the relationship between the time required to carry out 1024 steps of head-data transfer and the printer speed.
Under the conditions noted above, assume for simplicity that a clock cycle operating at 14 MHz is 72 ns (actually 71.4285 . . . ), then one step of head-data transfer requires 72 nsxc3x97128 dots=9.216 xcexcs. To switch head-data 1024-times requires 9.216 xcexcsxc3x971024=9438 xcexcs, indicating that it requires 9438 xcexcs to transfer a set of data for one line. Therefore, even if the printer is capable of printing one line in a minimum time of 2800 xcexcs, because the data transfer duration (9438 xcexcs) greater than one line printing duration (2800 xcexcs), it can be seen that the printing speed is determined by the speed at which the data are being transferred.
Accordingly, in the method based on successive switching of data dependent on the levels of gradation, it is necessary to transfer to the thermal head as many on/off instructions as there are levels of gradations to be expressed, therefore the printing speed is governed by the speed of transferring head-data to thermal head. To shorten the data transfer interval, the number of parallel signal lines may be increased so as to increase the number of data that can be input in each transfer step, but in such a circuit design, the number of separate input terminal required at the thermal head also increases, leading to an increase in the scale of the control circuit.
(2) Method Based on Functional Head
In this method, control functions are provided on the thermal head so that a register having a plurality of bits and a comparison circuit are provided in the thermal head for each 1-bit-data so that heat on/off can be controlled externally by counter control signals. A general circuit configuration is shown in FIG. 7.
Thermal head shown in FIG. 7 is comprised by 2816-members of heating elements RA1xcx9cRA2816; and 44-IC control circuits IC-A0xcx9cIC-A43, each of which controls the widths of pulses to be applied to each of the 64-pieces of heating elements according to 8-bit gradation level signals input from data signal lines DATA-0xcx9cDATA-7. Data signal lines DATA-0xcx9cDATA-7 are connected in parallel to a line joining the IC control circuits IC-A0xcx9cIC-A43, and each group of 11-Integrated circuits IC-A0xcx9cIC-10, IC-A11xcx9cIC-21, IC-A22xcx9cIC-32, IC-A33xcx9cIC-43 are controlled by respective control signals BE01, BE02, BE03, BE04. Read-data can be input only when the control signals BE01, BE02, BE03, BE04 are at xe2x80x9c1xe2x80x9d.
Each of the IC control circuits IC-A0xcx9cIC-A43 is provided with 64-cells of 8-bit registers, counter circuits, comparison circuits and the like so that clock signals are counted, and register selection and generation of comparison values are performed internally within the thermal head. The timing of data input/output operations is controlled by strobe signal STROBE and latch signal LATCH.
In such a method, on/off information for each gradation level of each heating element is transferred through data buses comprised by 8 lines of data signal lines DATA-0xcx9cDATA-7, as multiple bit gradation data, to each of the IC control circuits IC-A0xcx9cIC-A43. After the first group of control data are output, subsequent groups of control data are output by controlling the on/off information for each heating element according to the gradation levels determined by the gradation control counter provided within each integrated circuit.
According to this method, because the multi-bit gradation level data can be output in parallel, it is possible to shorten the data transfer interval compared with the method based on transmitting on/off information 1-bit at a time serially. However, because it is necessary to provide control circuits having gradation control functions on the head-side, not only the unit cost of Integrated circuits and the size of the control circuit increase but other problems are introduced such as the large size of thermal head and associated high cost.
It is an object of the present invention to provide a control device for a thermal printing head having one register holding 1-bit for heating each dot so as to produce multi-level gradation printing at a high speed, and to provide a printing apparatus having the control device according to the present invention for a thermal printing head.
The object has been achieved in the present invention in a control device for controlling a thermal printing head having a plurality of heating elements to correspond to a plurality of imprinting dots and a plurality of registers for holding individual 1-bit signals to drive respective heating elements, comprising dividing means for dividing multi-level gradation data expressing shading tones of imprinted dots into a plurality of different weighted values according to gradation values; and pulse signal generation means for generating a plurality of pulse signals having pre-determined pulse widths according to weighting of respective gradation values to be expressed so as to produce a number of pulses or pulse widths in accordance with a plurality of values of divided gradation data.
Accordingly, using a thermal head having 1-bit register for each dot, the present control device enables to produce multi-level gradation printing according to a data transfer process involving lesser number of transfer steps than the number of gradation levels to be expressed. Therefore, the present invention brings a benefit that it is possible to provide highspeed and high fidelity printing of patterns of complex tones using a low cost (commonly available) thermal printing head.
In addition, the object has been achieved in the present invention in a control device for controlling a thermal printing head having a plurality of heating elements to correspond to a plurality of imprinting dots and a plurality of registers for holding individual 1-bit signals to drive respective heating elements, comprising dividing means for dividing multi-level gradation data expressing shading tones of imprinted dots into a plurality of different weighted values according to gradation values; and pulse signal generation means for generating a plurality of pulse signals having pre-determined pulse widths according to weighting of respective gradation values to be expressed so as to produce a number of pulses or pulse widths in accordance with a plurality of values of divided gradation data, wherein pulse-on duration and pulse-off duration for impressing pulse signals during the pre-determined pulse widths are adjusted so that the temperature rise of the thermal printing head for one gradation level will be a constant value.
Accordingly, the pulse-on and pulse-off durations of the impressed pulse in a given pulse width are adjusted so that the temperature rise of the thermal printing head corresponding to one gradation level will be a constant value. By so doing, it is possible to prevent rapid temperature rise or rapid temperature decrease of the printing head. In addition, by controlling the head temperature to rise slowly at a constant rate, beneficial effects are realized such that degradation in the quality of the printed medium, caused by such factors as loss of gloss and feel of roughness of the paper surface brought about by a rapid increase in the head temperature is prevented. Also, if an ink ribbon is being used, it is possible to prevent severing of the ribbon due to heat effects. Also, by selecting the pulse-off duration as described above, excessive cooling of the printing head is prevented, thereby avoiding a situation that the necessary degree of print darkening is not obtained. Further, by preventing the head temperature from rising too rapidly, it is no longer necessary to provide customary thermal history control to counter rapid temperature changes, thereby enabling reductions in the cost and the size of the printer. Accordingly, it is possible to significantly increase the quality of imprinting by choosing the pulse-on duration and pulse-off duration of the pulse impression in a given pulse width such that the temperature rise of the thermal printing head for one gradation level will be a constant value.